The present invention generally pertains to video information signal processing and is particularly directed to storage and retrieval of video information in a slow scan television system.
Slow scan techniques were developed to accommodate high data rate sources to narrow band communication channels. Slow scan techniques have had particular application to the transmission of video information, such as a picture developed as a television type raster scan frame, over voice grade communication channels at a data rate that can be accommodated in the limited bandwidth available. The time necessary to transmit sufficient information for a video presentation of acceptable quality requires on the order of 8 to 32 seconds per frame. Such slow frame rates are incompatible with conventional television monitors because the phosphors utilized on the face of the display tube do not have sufficient persistence to stay illuminated for a full frame.
One type of slow scan television system utilizes a specially designed slow scan display tube incorporating long persistence phosphors. In such a tube, the illumination produced by the electron beam produces an after glow of the long persistence phosphors that permits viewing of a complete frame in relatively low ambient light conditions. The face of the tube is the only storage for the received image and it is therefore necessary to have a continuously repeating image for longer display. On such tubes, the writing line, or bright portion of the tube, makes it difficult to view the remainder of the tube. Further, the 8 second delay in production of a new picture makes it difficult to adjust the system for proper contrast and other picture variables.
The disadvantages associated with the use of long persistence phosphors has led to the development of other slow scan television systems that incorporate storage for a complete frame of video information separate from the display tube. A slow scan image is written into a memory until a completed picture is developed. The completed image may then be retrieved from the memory as many times as is desired at a fast scan rate and displayed on a conventional television monitor. However, such systems are not capable of simultaneously monitoring the stored image and storing/retrieving slow scan information.
Various systems have been proposed incorporating a second memory unit so that fast scan monitoring and simultaneous slow scan read/write functions may be obtained. However, such systems are excessively expensive for many applications and increase the complexity of the system thereby degrading reliability and operational flexibility.
To overcome the foregoing problems, the prior art has proposed the use of a high speed digital memory for storing incoming video information at both a slow scan rate and a fast scan rate and for retrieving the stored video information at both the slow rate and the fast scan rate. In these prior art systems, the incoming video information is "quantized" both in space (two dimensions X and Y), and in gray shades. More particularly, each line of the incoming video signal is divided into a plurality (typically 128) of picture elements or pixels each of which represent the relative brightness of a different location on the respective horizontal line of video information. The total number of pixels in a single frame of the input video signal (the number of pixels per line times the number of lines in the frame) are stored in a digital memory, and cumulatively represent the entire video information of the single video frame. A scan converter utilizing the foregoing system is disclosed in an article by Steber, "SSTV to Fast Scan Converter", Parts I and II, QST, March 1975, pp. 33-40 and May 1975, pp. 28-46. A similar system is disclosed in U.S. Pat. No. 4,057,836. Another system utilizing an encoding technique is described in a second article by Steber, "SSTV Image Processing", QST, November 1976, pp. 13-16.
While the foregoing systems have produced generally satisfactory results, they are quite expensive. In order to perform the digital scan conversion, it is necessary to digitally encode the brightness of each pixel in the entire video frame. For good picture quality, it has been established that at least 64 brightness levels (six digital bits) per pixel are necessary since the eye is very sensitive to small brightness introduced by quantization. If less than six bits are used an objectionable phenomena called "contouring" becomes apparent to the viewer.
Since a typical horizontal slow scan line consists of 128 pixels and there are nominally 128 lines per frame, the stored video picture consists of 16,384 pixels. Since six digital bits are required per pixel, a total of 98,304 bits must be stored in the memory of the scan converter. If less bits per pixel could be used while still remaining the good picture quality, a substantial savings in the cost of the memory may be obtained.